The conversion of sea water into fresh water for use as irrigation water has been studied for the greening of deserts or the cultivation of agricultural crops in arid regions, and the use of fresh water generators such as those depicted in FIGS. 5 and 6 have been proposed for irrigation facilities. These are systems in which a single distilling tube 1, formed using a vapor-permeable membrane having the physical property of liquid water impermeability but allowing water vapor to permeate, is buried in the ground in regions that are to be irrigated. Sea water 2 is stored in a tank 3 where it is typically heated to about 50 degrees C. by a solar collector 4, for example, and the heated sea water 2 is fed under pressure by a pump 5 and circulated to the distilling tube 1. In this fresh water generator, simply heating the sea water 2 and feeding it under pressure to the distilling tube 1 allows the sea water 2 to be converted to vapor as a result of the difference between the temperature of the heated sea water itself and the temperature in the surrounding ground. The vapor permeates the material comprising the distilling tube 1 and is exuded into the ground, where it is immediately cooled and condensed, thus watering the ground in the form of distilled water, that is, fresh water. Fresh water is thus naturally generated by the aforementioned distilling tube 1 and immediately supplied into the ground. The salt component and other components in the sea water 2 cannot permeate the distilling tube 1 and are thus gradually concentrated and collected in tank 3.
Because the raw water 2 is heated to about 50 degrees C. and fed under pressure to the distilling tube 1 in the aforementioned fresh water generator, the temperature in the ground surrounding the distilling tube 1 gradually increases to nearly the same temperature as the feed water 2, leading to concerns over the deleterious heating effects on the roots of the crops as well as the other drawback that adequate distillation efficiency (driving force resulting from actual temperature difference) is not necessarily achieved.
The distillation efficiency of the distilling tube 1 is affected by the difference between the temperature of the sea water 2 and the temperature in the ground surrounding the distilling tube 1. The greater the difference in temperature, the more efficient is the distillation. Although an adequate difference in temperature is maintained initially in the known fresh water generator, continuous long term operation results in the gradual increase of the temperature in the ground surrounding the distilling tube 1 and the gradual decrease in the difference in temperature with the sea water 2, ultimately leading to markedly diminished distillation efficiency and the inability to obtain adequate amounts of fresh water for supply. As a consequence, the aforementioned prior device must be run intermittently by stopping operations when the temperature in the surrounding ground has increased sufficiently and the distillation efficiency has decreased, waiting for the surroundings to cool off sufficiently, and then resuming operations. This system thus cannot maintain demand when large amounts of fresh water must be supplied in a short period of time, and the overall distillation efficiency is not adequate. Such distillation apparatus is disclosed, for example, in European Patent Application No. 871035663, filed Mar. 3, 1987.
Other devices for producing fresh water from salt water or otherwise contaminated water are disclosed in U.S. Pat. Nos. 4,698,135; 4,666,883; 4,596,659 and 4,178,715.